Emission concerns associated with the operation of internal combustion engines, generally, but not always, diesel engines, have resulted in an increased emphasis on the use of exhaust gas heat exchange systems with such engines, particularly, but not always, in vehicular applications. These systems are employed as part of an exhaust gas recirculation (EGR) system by which a portion of an engine's exhaust is returned to its combustion chambers via its intake system. The result is that some of the oxygen that would ordinarily be inducted into the engine as part of its fresh combustion air charge is displaced with inert gases thus reducing the rate of NOx formation. EGR systems are frequently designed to recirculate the cooled exhaust gas, thus lowering the combustion temperature and providing a reduction in NOx.
In many applications employing EGR systems, exhaust gas recirculation coolers are employed. In the usual case, engine coolant is brought into heat exchange relation with the exhaust gas to lower its temperature prior to recirculation. Conventional EGR coolers use a single coolant loop to cool the exhaust gas. Most of the heat is absorbed by the coolant over the first few inches of the cooler. This raises the coolant temperature and makes the heat exchange less effective over the length of the cooler as the differential temperature between the coolant and exhaust gases become smaller.
The present invention is directed to improvements in EGR cooler operation to further reduce exhaust gas outlet temperatures.